The gallium nitride (GaN)-based Group III-V compound semiconductors of the cubic zinc blende crystal or the hexagonal wurtzite crystal have been heretofore utilized for fabricating short wavelength visible light-emitting devices, for example (refer, for example, to JP-A HEI 2-288388). The stacked structures used for the fabrication of gallium nitride-based semiconductor devices have been manufactured by using single crystals of aluminum oxides having high thermal resistance, such as a solid-state single crystal of sapphire (α-Al2O3 single crystal) or garnet, as a substrate (refer, for example, to JP-A HEI 7-288231).
However, the single crystal substrates formed of aluminum oxides, such as sapphire, and the gallium nitride (GaN)-based semiconductor materials have profoundly different lattice constants. The lattice mismatch factor between sapphire and hexagonal GaN, for example, is as large as 13.8% (refer, for example, to Kazumasa HIRAMATSU (et al.), “Journal of Japanese Association for Crystal Growth,” Vol. 20, No. 4, 1993 (Japan), pp. 28-36). Thus, the stacked structures used for fabricating gallium nitride-based semiconductor devices are generally formed on their substrates through a buffer layer deposited thereon. The buffer layers aimed at relaxing lattice mismatch have been heretofore formed at a comparatively low temperature and, therefore, are called low-temperature buffer layers (refer, for example, to Isamu AKASAKI, “Group III-V Compound Semiconductors,” published on May 20, 1995, by Baifukan K. K., first edition, Chapter 13).
The low-temperature buffer layers are formed of gallium nitride, for example (refer, for example, to JP-A HEI 8-255926). It is held that these low-temperature buffer layers are preferably formed of an amorphous or polycrystalline material in an as-grown state for the sake of relaxing the lattice mismatch with the crystals of the substrate (refer, for example, to JP-A HEI 8-255926). In fact, the p-n junction-type blue LED provided with a light-emitting layer formed of gallium nitride and making use of a low-temperature buffer layer of aluminum nitride (AlN) having such a structure has been reduced to practice (refer, for example, to “Electronics,” March 1991 issue, pp. 63-66). Further, the fact that the low-temperature buffer layers of this structure are utilized in short wavelength LED products emitting a blue color band light and an ultraviolet band light has been disclosed in early publications, such as Hiroshi AMANO and Isamu AKASAKI, “Solid Physics,” Vol. 25, No. 6, 1990 (Japan), pp. 35-41 and H. AMANO (et al.), Institute of Physics Conference Series, Vol. 106, Chapter 10, 1990 (England), pp. 725-730. This fact pertaining to MIS-type blue LED products has been disclosed in earlier publications, such as Masahide MABE, “TOYODA GOSEI TECHNICAL REVIEW,” Vol. 31, No. 2, 1989 (Japan), pp. 85-93 (particularly FIG. 1 in the left column, page 87 and lines 5-6 from the top of the left column).
When the low-temperature buffer layer constitutes itself an under layer, it is regarded as exhibiting superiority in forming thereon a semiconductor layer of a Group III nitride, such as GaN, which excels in continuity at a temperature higher than the temperature used for growing the low-temperature buffer layer. This superiority is interpreted as caused by the presence of the low-temperature buffer layer, which promotes the growth in the lateral (horizontal) direction of the single crystal grains of the Group III nitride semiconductor occurring on the surface thereof and consequently allows smooth advance of the cohesion of the Group III nitride semiconductor single crystal grains prevalently developing in the horizontal direction (refer, for example, to Isamu AKASAKI, Hiroshi AMANO, Yasuo KOIDE, Kazumasa HIRAMATSU and Nobuhiko SAWAKI, Journal of Crystal Growth, Vol. 98, 1989 (The Netherlands), pp. 209-219). The mutual cohesion of the Group III nitride semiconductor single crystal grains which induces the continuity, when the single crystal grains are hexagonal, can occur in the (10-10) plane in the Miller≅Bravais index and the face crystallographically equivalent thereto.
For example, in the low-temperature buffer layer formed of a Group III nitride semiconductor on the sapphire with a (0001) surface (so-called C surface) using a substrate, the Group III nitride semiconductor low-temperature buffer layer having a single crystal layer formed of a Group III nitride semiconductor, such as of Al2GaγN in the junction region with the substrate, has been demonstrated to be effective in inducing a gallium nitride-based semiconductor layer oriented uniformly in a specific crystal direction (refer, for example, to JP-A-HEI-10-022224).
Besides the uniform orientation, the structural requirements which the low-temperature buffer layer formed of a Group III nitride semiconductor ought to fulfill for the purpose of inducing as the upper layer a gallium nitride-based semiconductor layer suffering crystal defects, such as stacking faults, only at a low density and excelling in single crystallinity have not been satisfactorily elucidated. Thus, the conventional low-temperature buffer layers, notwithstanding their uniform orientation, still constitute themselves under layers which fall short of inducing fully stably a gallium nitride-based semiconductor layer suffering crystal defects only in a sufficiently low density and excelling in crystallinity as an upper layer.
This invention has been initiated with a view to overcoming the problems encountered by the prior art as described above and is aimed at providing a gallium nitride-based semiconductor stacked structure provided with a buffer layer as an under layer (substratum) capable of fully stably inducing a gallium nitride-based semiconductor stacked structure to acquire uniform orientation, suffer crystal defects at a fully lowered density and excel in crystallinity, a method for the production thereof, and gallium nitride-based semiconductor device and lamp.